This application is based upon and claims priority of Japanese Patent Application No. 10-369006 filed Dec. 25, 1998, the contents being incorporated herein by reference.
1. Field of the Invention
The present invention relates to a DC-to-DC converter circuit including a main switching element to perform DC-to-DC conversion by using the main switching element to turn an input voltage on and off. More specifically, the present invention relates to a DC-to-DC converter circuit that achieves a high conversion efficiency without using a sense resistance.
2. Description of the Related Art
Battery-driven devices, such as notebook personal computers (PCs), typically include DC-to-DC converter circuits to convert the voltage of AC adapters, dry-cell batteries and the like into a voltage required by a load. In order to increase the utility of such battery-driven devices, the conversion efficiency of the DC-to-DC converter circuit must be increased.
The conversion efficiency of conventional DC-to-DC converter circuits provided in battery-driven devices, such as notebook PCs, is made as high as possible using a switching regulator to perform pulse width modulation (PWM) control. DC-to-DC converter circuits using the type of switching element that performs PWM control can be either a voltage mode control DC-to-DC converter circuit or a current mode control DC-to-DC converter circuit, depending on the method of control.
FIG. 31 illustrates a conventional voltage mode control DC-to-DC converter circuit. As shown in FIG. 31, the voltage mode control DC-to-DC converter circuit generates a PWM control signal (Vpwm) and comprises a triangular wave generation circuit 2 to generate triangular wave signals, an error amp (AMP) to output a voltage Ver in response to an output voltage Vout, and a comparator (COMP) to compare the triangular wave signals output by the triangular wave generation circuit 2 and an output voltage Ver of the error amp (AMP). The DC-to-DC conversion circuit shown in FIG. 31 performs DC-to-DC conversion by turning a main switching element Q1 on and off via a driver circuit 4.
The conventional voltage mode control DC-to-DC converter circuit shown in FIG. 31 includes a synchronous commutating switching element Q2 in place of a flywheel diode, and a main switching element Q1. The ON/OFF operation of the synchronous commutating switching element Q2 is performed as a reverse operation of the ON/OFF operation of the main switching element Q1. When the main switching element Q1 is off, the synchronous commutating switching element Q2 supplies current to an output capacitor C1 from an inductor L1 with a smaller drop in voltage than with a flywheel diode.
An example of a conventional current mode control DC-to-DC converter circuit is the MAX 786 PWM controller made by the Maxim Co. of the U.S. As shown in FIG. 32, the MAX 786 PWM current mode control DC-to-DC converter circuit comprises a sense resistance R to detect a load voltage; an error amp (AMP 1) to output a voltage in response to an output voltage Vout; a current amp (AMP 2) to receive the voltage sensed by the sense resistance R and to output a voltage that becomes larger as the input voltage generated by sense resistance R becomes larger; a current comparator (COMP 1) to compare the output of the error amp AMP 1 with the output of the current amp AMP 2, and to output a high level when the current amp AMP 2 output voltage reaches an output voltage Ver of the error amp AMP 1; and a flip-flop FF1 to latch a high level in response to a predetermined frequency pulse and to reset the latch output at a low level when the current comparator COMP 1 outputs a high level. A control logic circuit 6 turns the main switching element Q1 on and the synchronous commutating switching element Q2 off when the flip-flop FF1 outputs a high level, and turns the main switching element Q1 off and the synchronous commutating switching element Q2 on when the flip-flop FF1 outputs a low level.
As shown in FIG. 32, a reverse current comparator (COMP 2) receives the voltage output by the sense resistance R, detects the reverse current generated when the load current becomes small (that is, the current flowing from capacitor C1 to inductor L1) and outputs a high level. A flip-flop FF2 latches the high level when the reverse current comparator (COMP 2) outputs a high level, and resets the latch output at a low level in response to the pulse input by the flip-flop FF1. When the flip-flop FF2 indicates the generation of a reverse current, the control logic 6 then cuts off the reverse current by turning synchronous commutating switching element Q2 off in order to prevent wasted consumption of power by the sense resistance R.
Further, a mini-current comparator (COMP 3) receives the output voltage of the current amp AMP 2 as an input, detects when the load current is even smaller than the level generated by the reverse current, and outputs a high level. When the mini-current comparator COMP 3 detects a decrease in the load current, the control logic 6 enters a power saving mode (pulse skip mode). The drive current of the main switching element Q1 (synchronous commutating switching element Q2), which is problematic when the charge current is low, can be reduced by selecting the drive instruction signals of the main switching element Q1 (synchronous commutating switching element Q2) input from the flip-flop FF1.
In operation of the device shown in FIG. 32, when entering the power saving mode, after extra energy is injected into an output LC filter by turning the main switching element Q1 on at the maximum duty on width, the power saving mode is entered by causing the main switching element signal Q1 and the synchronous commutating switching element Q2 to rest.
In the above-described manner, the current mode control DC-to-DC converter circuit shown in FIG. 32 functions to stop the reverse current by turning the synchronous commutating switching element Q2 off when the reverse current is generated in response to the load current becoming smaller, thereby preventing the waste of electric power caused by the reverse current flowing through the sense resistance R.
The current mode control DC-to-DC converter circuit also has the function of reducing the drive current of the main switching element Q1 (synchronous commutating switching element Q2) when the load current becomes small, which becomes problematic when the load current becomes small, by selecting the drive instruction signals of the main switching element Q1 (synchronous commutating switching element Q2).
On the other hand, unlike the current mode control DC-to-DC converter circuit, the voltage mode control DC-to-DC converter circuit shown in FIG. 31 is unable to improve the conversion efficiency because it does not have the ability to measure the load current. Therefore, when a high conversion efficiency is required, the current mode control DC-to-DC converter circuit has been employed.
However, the current mode control DC-to-DC converter circuit shown in FIG. 32 has the problem of wasteful use of energy by the sense resistance since the sense resistance is used to measure the load current.
Recently, the load current in notebook Pcs having DC-to-DC converter circuits has been constantly increasing as functions become more and more advanced, and it is now impossible to ignore the loss of power caused by the sense resistance. For example, when a sense resistance of 22 mxcexa9 is used, if the load current is 4 A, the power loss caused by the sense resistance is 22 mxcexa9xc3x974 A2=0.352 W. With an output of 3.3 V this becomes a power loss of 2.67%.
Moreover, the sense resistance has the problem of high price because it is a special article with low resistance in the tens of mxcexa9 and precision below 1%.
In order to solve the above-described problems, technology has been disclosed that uses the xe2x80x9con xe2x80x9d resistance of the main switching element Q1 in place of the sense resistance. However, when the xe2x80x9conxe2x80x9d resistance of the main switching element Q1 is used as the sense resistance , a different problem occurs because large restrictions on design are imposed as the range of choices for the main switching element Q1 disappears.
It is an object of the present invention to overcome the above-described problems of the prior art DC-to-DC converter circuits.
It is another object of the present invention to provide a DC-to-DC converter circuit to perform a highly efficient conversion without using a sense resistance.
It is yet another object of the present invention to provide a DC-to-DC converter circuit to perform DC-to-DC conversion by using switching elements to turn an input voltage on and off.
Objects and advantages of the present invention are achieved in accordance with embodiments of the present invention with a DC-to-DC converter circuit comprising: a triangular wave generation circuit to generate a triangular wave signal; a differential amplifier to receive the triangular wave signal and to generate an output signal; a main switching element to turn on and off an input voltage based on an operation control signal generated in response to the triangular wave signal and the output signal of the differential amplifier; a synchronous commutating switching element to perform synchronous commutation of a load current and having an off period simultaneous with that of the main switching element, and which turns on and off in a mode opposite to that of the main switching element; a detection device to detect whether an output voltage of the main switching element is larger than an input voltage of the main switching element; and a control device to operate in a power saving mode to reduce a drive voltage of the main switching element in response to the detection device detecting that the output voltage of the main switching element is larger than the input voltage.
In accordance with embodiments of the present invention, the control device performs control to operate in the power saving mode, which results in a low consumption of energy. Specifically, the control device eliminates the drive of the synchronous commutating switching element by turning the synchronous commutating switching element off. Alternatively, the control device may eliminate the drive of the main switching element and the synchronous commutating switching element by turning the main switching element and the synchronous commutating switching element off; or the control device may reduce the drive frequency of the main switching element and the synchronous commutating switching element by lowering an oscillating frequency of the triangular wave signal; or the control device may reduce the drive frequency of the main switching element and eliminate the drive of the synchronous commutating switching element by lowering the oscillating frequency of the triangular wave signal and by turning the synchronous commutating switching element off.
In accordance with embodiments of the present invention, the DC-to-DC converter further comprises an output filter including an inductor. The current flowing to the inductor of the output filter becomes lower than 0 A (enters a reverse current state) when the load current is small, and thus the current flows into the inductor. If this phenomenon occurs when the synchronous commutating switching element is on (a condition permitting reverse current), and the main switching element and the synchronous commutating switching element are then turned off, the current that flowed into the inductor flows back toward the input voltage via a parasitic diode of the main switching element, thus causing the output voltage of the main switching element to an become larger than the input voltage. In other words, when the load current becomes small, the phenomenon occurs that the output voltage of the main switching element becomes larger than the input voltage.
The detection device detects whether the load current has become small by detecting whether or not the output voltage of the main switching element has become larger than the input voltage. In response to the detection device detecting that the load current has become small, the control device performs control to operate the DC-to-DC converter circuit in the power saving mode, which results in a low consumption of power.
In accordance with embodiments of the present invention, the DC-to-DC converter circuit detects when the load current has become small without the use of a sense resistance. In response to detecting that the load current has become small, the power saving mode is entered, which lowers the drive frequency of the switching elements. Thus, the low power consumption that is desirable when the load current becomes small is achieved without the use of sense resistance. Further, by eliminating the sense resistance to reduce the consumption of power, a highly efficient conversion is achieved, as well as cost reduction.
In accordance with embodiments of the present invention, when a power saving mode in which the main switching element and synchronous commutating switching element are turned OFF is used, the DC-to-DC converter circuit cancels the power saving mode when the lowering of the output voltage becomes excessive by providing, since turning the main switching element off lowers the output voltage, 1) a detection device to detect whether or not the output voltage has become smaller than a predetermined value and 2) a cancellation device to cancel the power saving mode in response to the detection device detecting a lowering of output voltage. Alternatively, in accordance with the present invention, the DC-to-DC converter circuit cancels the power saving mode when the lowering of the output voltage is excessive by providing, since the output voltage of the differential amplifier varies in a predetermined direction in response to the lowering of the output voltage, 1) a detection device to detect whether or not the output voltage of the differential amplifier has varied such that it has reached a predetermined value, and 2) a cancellation device to cancel the power saving mode in response to detection device detecting that the output voltage of the differential amplifier has varied.
Further, in accordance with the present invention, when a power saving mode is used in which the main switching element is turned on and off while the synchronous commutating switching element is turned off, the power saving mode is cancelled when the load current becomes large by providing, since the output voltage of the differential amplifier is modified in a predetermined direction as a result of the inductor current becoming non-continuous when the load current is small, 1) a detection device to detect whether or not the output voltage of the differential amplifier has changed to reach a predetermined value, and 2) a cancellation device to cancel the power saving mode in response to the detection device detecting a change of the output voltage of the differential amplifier.
Further, in accordance with the present invention, when a power saving mode is used in which the main switching element is continuously turned on and off, the power saving mode is cancelled when the load current becomes large by providing, since the peak value of the inductor current becomes large when the load current becomes large, 1) a detection device to detect whether or not the peak value of the current that flows to the inductor which is part of the output filter has become larger than a predetermined value by using the on resistance, or the like, of the main switching element, and 2) a cancellation device to cancel the power saving mode in response to the detection device detecting a rise in the peak value of the inductor current.
In accordance with the present invention, when a power saving mode is used in which the main switching element and the synchronous commutating switching element are turned off, it is preferable to provide a cancellation control device to control such that the main switching element is the first to be turned ON in response to the cancellation device cancelling the power saving mode, so that the energy stored in a capacitor which is part of the output filter does not dissipate.
Further, in accordance with the present invention, when a power saving mode is used in which the main switching element and the synchronous commutating switching element are turned off, an injection device injects energy into the capacitor which is part of the output filter in response to the cancellation device canceling the power saving mode, since the energy stored in the capacitor which is part of the output filter is low.
The injection device injects energy into the capacitor which is part of the output filter by making the on width time prescribed by the operation control signal larger than usual (for example, the ON width time can be modified in correspondence with the input voltage), or by keeping the main switching element on.
Furthermore, a stop device to stop the energy injection processing executed by the injection device is provided. The stop device stops the energy injection processing by causing the energy injection to stop when the output voltage has become larger than a predetermined value, since the output voltage rises as a result of the energy injection processing. Alternatively, the stop device stops the energy injection processing by causing the energy injection to stop when the output voltage of the differential amplifier has changed such that it has reached a predetermined value (it can be modified in correspondence with the input voltage), since the output voltage of the differential amplifier is modified in a predetermined direction in response to a rise in the output voltage. Alternatively, the stop device can stop the energy injection processing by using the on resistance of the main switching element, or the like, to detect a peak value of the current flowing to the main switching element, and causing the energy injection processing to stop when the peak value is larger than a predetermined value.
When stopping the energy injection processing, the stop device stops the energy injection processing by turning only the main switching element off. Alternatively, the stop device may stop the energy injection processing by turning the main switching element and the synchronous commutating switching element off.
In accordance with embodiments of the present invention, the DC-to-DC converter circuit detects when the load current has become small without the use of sense resistance. When it is detected that the load current has become small, the power saving mode is entered, which lowers the drive frequency of the switching elements. Thus, the low power consumption, which is desirable when the load current becomes small, is achieved without the use of sense resistance. Thus, by reducing the consumption of power by a sense resistance, a highly efficient conversion is achieved, as well as cost reduction.
In accordance with embodiments of the present invention, the conditions for canceling the power saving mode of the DC-to-DC converter circuit can be accurately detected.
In accordance with embodiments of the present invention, when the power saving mode of the DC-to-DC converter circuit is cancelled, energy, which had decreased as a result of entering the power saving mode, is injected in a short span of time into the output filter. Furthermore, in accordance with the present invention, the conditions for stopping the injection of energy can be accurately detected.